To feed electrical energy generated with DC voltage generators such as photovoltaic or fuel cell plants into an AC grid, in particular into the utility grid (50/60 Hz), various inverters are used. Between the DC voltage generator and the inverter there is mostly provided a DC voltage converter (DC-DC chopper) that serves the purpose of converting the DC voltage supplied by the DC voltage generator into a DC voltage needed by the inverter or adapted thereto.
For different reasons, it is desired to ground one of the outputs of the DC voltage generator. The reason for the desired grounding is on the one side that there are countries which prescribe such grounding. On the other side, different disadvantages arise in operation when grounding is missing. One of the problems encountered is that of the high-frequency leakage currents. Due to inevitable, parasitic capacitances between the DC voltage generator and the ground, considerable equalizing currents creating an intolerable safety risk may occur in case of potential fluctuations so that complex monitoring measures using fault current sensors or the like are needed for contact protection or for establishing the electromagnetic compatibility (EMC) and said equalizing currents can only be securely avoided by grounding. Moreover, it is known that photovoltaic generators behave very differently with respect to degradation, depending on which technology is used to manufacture them. Generators with crystalline and polycrystalline cells or certain thin film modules are preferably grounded with the negative terminal, whilst backside-contact cells are preferably grounded at the positive terminal.
A grounding of the type described, through which the disadvantages mentioned could be avoided, is readily possible using DC voltage converters with transformers which cause the DC voltage side to galvanically separate from the AC voltage side. Irrespective of whether grid transformers or high-frequency transformers are being used, transformers result i.a. in a reduction of efficiency, in parts in considerable weight and overall size and/or in an additional control expense, which is the reason why transformerless voltage converters are in principle preferred. However, the usual topologies of transformerless DC voltage transformers either make the desired grounding impossible to perform since this grounding would lead to a short-circuit of needed switches, capacitances or the like or they result in increased circuit expense and other disadvantages.
Therefore, numerous attempts have been made to avoid the mentioned disadvantages in another way. Circuits are known in particular, which serve the purpose of reducing the undesirable leakage currents (e.g., DE 10 2004 037 466 A1, DE 102 21 592 A1, DE 10 2004 030 912 B3). In these circuits, a solar generator e.g., is operated isolated from the grid in certain phases of internal electrical energy transport. When the solar generator is periodically electrically reconnected to the grid, its parasitic capacitances are only slightly reconverted so that the potential of the solar generator changes with grid frequency, sinusoidally and at a voltage amplitude that corresponds to half the grid voltage. High-frequency currents then form through the low voltage differences of the solar generator between two switching cycles only and through asymmetries during switching. Capacitive leakage currents can thus be strongly minimized but cannot be avoided completely as a matter of principle.
A circuit arrangement is further known (DE 102 25 020 A1), which uses a divided solar generator the center point of which is grounded. As a result, all the parts of the solar generator have a fixed potential and capacitive leakage currents cannot flow in principle. Since the two direct current sources have a different yield, a circuit for compensating the power differences and the voltages is additionally provided. In this proposed circuit, the disadvantage lies in the high voltage differences in the solar generator and at the switches, in the additional losses in the compensation circuit and in the fact that at least four high-frequency pulsed switches are needed.
Besides, circuit arrangements have already been known by means of which a solar generator can be grounded on one side, in spite of the lack of a transformer. As a result, capacitive leakage currents are prevented as a matter of principle. One of these circuit arrangements (DE 196 42 522 C1) however needs five active switches, one or two switches having to switch simultaneously at high frequency and to provide the average output current. With this circuit, which is also referred to as “Flying Inductor”, the efficiency is therefore affected by the high number of components participating simultaneously in series in the current flow. Another disadvantage of this circuit is that discontinuous current pulses are impressed upon the grid, which call for a capacitive mains filter which, as inherent to its functional principle, degrades the power factor but also the efficiency of the circuit in the part load range because of its own need for idle power. Although such a capacitive mains filter can be avoided with another known circuit (DE 197 32 218 C1), nine active switches are needed therefor, of which at least two must be switched simultaneously at high frequencies so that the expense in terms of construction is even further increased and both the robustness and the efficiency of the overall apparatus negatively affected. The topology of a Flying Inductor also has the disadvantage that the voltage load of the switches depends on the grid voltage and is sensitive to mains power failures and that it can only be operated in the three-phase mode of operation by three-fold use with the help of three inverters. Irrespective thereof, inverters with current source characteristics are needed, which is undesirable in many cases.
Finally, apparatus are known (US 2007/0047277 A1), which are intended for inverters having a bipolar voltage intermediate circuit containing two series-connected capacitors connected together at a ground terminal. Such type inverters, which are nowadays mainly used for the purposes of interest herein, can be configured as half-bridge inverters in 3-level circuits and at need as inverters for one-phase or three-phase grid supply. In all of these cases, the connection node between the two capacitors forms a ground terminal that is associated with the zero or neutral conductor of the respective grid and is connected therewith.
The DC voltage converter of this known apparatus contains one choke, two diodes and one switch. In this case, the ground terminal of the inverter can be connected to the negative output of the DC voltage generator. This is made possible using a storage choke that is composed of two magnetically coupled coils. The two coils of this storage choke are galvanically connected together at one end in such a manner that on the one side, when the switch is closed, one of the two coils is charged by the DC voltage generator and the other coil via the first coil by virtue of the magnetic coupling and that, on the other side, when the switch is open, the two coils are discharged via a respective one, associated, of the two capacitors and via a diode belonging thereto.
The advantage that this apparatus makes it possible to ground the DC voltage generator with relatively simple means, in particular without any transformer and with only one switch, is offset by the disadvantage that the ground terminal can only be connected to the negative output of the DC voltage converter. Further, this apparatus does not allow for monitoring the ground line leading from the ground terminal to the DC voltage generator with respect to fault currents since, as a matter of principle, operating currents also flow in this ground line.
A circuit with a power storage choke and two switches connected in series therewith is known from JP 11 235024 A1. On the output voltage side, there are provided two diodes in order to decouple the input and the output. A DC-AC converter with a negative and a positive input and a three-phase AC output is used. Grounding is provided neither at the input nor at the output of the DC-AC converter. It is not mentioned whether the DC-AC converter is transformerless. At the output of the DC-DC circuit, there is only provided one single capacitor. Through this circuit, bidirectional operation of a DC-DC converter can be provided.